High frequency filter

1. A high frequency filter of coaxial construction, including at least one resonator comprising:

an electrically conductive internal conductor configured as an internal conductive tube;

an electrically conductive external conductors;

an electrically conductive base which electrically interconnects the internal conductor and the external conductors;

a cover covering at least the high frequency filter with respect to the base and having an inner side and outer side, the inner side pointing toward a free end of the internal conductive tube;

a dielectric layer having a relative dielectric constant greater than 2 arranged between the outer side of the cover and the free end of the internal conductive tube;

the radial extent of the dielectric layer substantially covering the cross section of the internal conductive tube at the free end thereof;

wherein

the dielectric layer is arranged on or fastened to the cover, and

wherein the dielectric layer is inserted in a recess in the inner side of the cover.

2. The high frequency filter as claimed in claim 1 , wherein the dielectric layer is held in the recess by an interlocking fit, by an edge, projecting beyond the edge of the dielectric layer, on the inner side of the cover.

3. The high frequency filter as claimed in claim 1 , wherein the dielectric layer is closed by the inner side of the cover.

4. The high frequency filter as claimed in claim 1 , wherein the dielectric layer is held on the inner side of the cover by an adhesion means.

5. The high frequency filter as claimed in claim 1 , wherein the relative dielectric constant of the dielectric layer is ≧5.

6. The high frequency filter as claimed in claim 1 , wherein the relative dielectric constant of the dielectric layer is ≧40.

7. The high frequency filter as claimed in claim 1 , wherein the dielectric layer comprises ceramic material.

8. The high frequency filter as claimed in claim 1 , wherein the surface area of the radial extent of the dielectric layer is at least twice the surface area of the cross section of the internal conductive tube at the free end thereof.

9. The high frequency filter as claimed in claim 1 , wherein the cross section of the internal conductive tube is substantially circular at the free end thereof.

10. The high frequency filter as claimed in claim 1 , wherein the radial extent of the dielectric layer is substantially circular.

11. The high frequency filter as claimed in claim 9 , wherein the diameter of the radial extent of the dielectric layer corresponds at least to the diameter of the cross section of the internal conductive tube at the free end thereof.

12. The high frequency filter as claimed in claim 11 , wherein the diameter of the radial extent of the dielectric layer is at least 1.5 times the diameter of the cross section of the internal conductive tube at the free end thereof.

13. The high frequency filter as claimed in claim 10 , wherein the external conductor is an external conductive tube having a substantially circular cross section and the diameter of the external conductive tube is at least twice the diameter of the radial extent of the dielectric layer.

14. The high frequency filter as claimed in claim 1 , wherein the high frequency filter comprises a plurality of resonators, a single continuous, at least partially strip-like dielectric layer being provided for all of the resonators.

15. The high frequency filter as claimed in claim 1 , wherein the resonators are configured and coupled in such a way that a duplex switch is formed.

16. The high frequency filter as claimed in claim 1 , wherein the resonators are configured and coupled in such a way as to provide band filter.

17. The filter of claim 5 wherein the dielectric constant is ≧9.

18. The filter of claim 6 wherein the dielectric constant is between 60 and 80.

19. The filter of claim 16 wherein the band filter comprises a band pass filter.